The present invention relates to an improved process for the manufacture of a rubber modified thermoplastic resin. The invention also encompasses a resin made by this process.
Impact-resistant thermoplastic resins are conventionally obtained by hot mixing a powder resulting from the steps of coagulation, dewatering and drying of an elastomeric latex with the particles of a thermoplastic resin or "hard" polymer, resulting in what is called an "alloy." This process is unsatisfactory regarding both economy and the quality of the alloy.
To overcome these disadvantages, processes have already been proposed in which the abovementioned dewatering and drying steps are eliminated. In these processes, an emulsion of the elastomeric latex is followed directly by the polymerization of the thermoplastic resin. Polymerization results from the addition of electrolytes, acids and unsaturated monomer to the elastomeric latex which, after polymerization, should produce the thermoplastic matrix. These electrolytes or acids are destablizing agents which neutralize the stablizing effect of an ionic surfactant present on the elastomeric latex, causing the latter to flocculate. When monomer is added after the destablizing agent, more or less rapid swelling by the monomer takes place. It is thus possible to end with a suspension polymerization. When the monomer is added before the destablizing agent, the latter will cause an instantaneous transfer of the latex particles into the monomer, resulting in an organic phase containing a rubbery dispersion, which can then be polymerized in suspension or in bulk. Such techniques are described in Japanese Patent Nos. 82-36,102, 78-44,959, 81-50,907, 74-02,347, 74-11,748, 74-11,749, 79-18,893, 75-31,598, U.S. Pat. Nos. 4,141,932, 3,450,796, and 3,950,455, and German Patent No. 2,524,471.
These processes, however, require the use of large quantities of coagulating agents. In fact, the examples of Japanese Patent No. 75-31,598 show that a proportion of approximatley 3% by weight of magnesium sulfate, relative to the final resin, is generally needed to destablize the latex. Moreover, suspension polymerization is difficult in the case of a quantity greater than 35% by weight, relative to the final resin, of elastomeric particles, comprising a surface graft of "hard" polymer (hereinafter "grafted particles"), owing to the viscosity of the solution of the monomer in which these particles are dispersed and swollen.
As a result, research has been carried out to propose a process for the production of rubber-modified, impact-resistant thermoplastic resins, substantially free from the disadvantages referred to above.